It is an object of the present invention to provide a device for adding or dropping optical signals of different wavelengths into or from an optical path and a method for inserting and/or extracting optical signals of different wavelengths into and from an optical path.
In the latest telecommunications technology, it is known to use optical fibers to send optical information-carrying signals for long-distance communication.
Optical telecommunication systems are known that use wavelength division multiplexing (WDM) transmission. In these systems several channels, i.e. a number of independent transmission signals, are sent over the same line by means of wavelength division multiplexing. The transmitted channels may be either digital or analog and are distinguishable because each of them is associated with a specific wavelength.
The Applicant has observed that known WDM communication systems are limited as concerns the number of channels, i.e. the independent wavelengths that can be used for transmission within the wavelength band available for signal transmission and amplification.
In order to combine and separate signals with different wavelengthsxe2x80x94to combine the signals at the transmission station, for example, to drop some toward receivers located at intermediate nodes of the line or to introduce others at intermediate nodes or to send them to separate receivers at the receiving stationxe2x80x94adjacent channels (in wavelength terms) must be separated by more than a minimum predetermined value.
Said minimum value depends on the characteristics of the components employed in the system, such as the spectral characteristics of the wavelength selective components (e.g. bandwidth, center-band attenuation, figure of merit) and the wavelength stability (thermal and temporal) of the selective components themselves and of optical signal sources.
In particular, the Applicant has observed that spectral selectivity of currently available wavelength selective components currently available may greatly limit the possibility of adding and dropping signals in multichannel transmission systems, particularly when there are signals with close wavelengths, e.g. separated by less than 2 nm.
The Applicant found it is possible to add and/or drop in an optical telecommunication system a number of independent optical channels greater than that permitted by known techniques, and closer in wavelength, by employing wavelength selective components of equivalent characteristics, if the input signals are divided into two series of staggered wavelengths. The signals of each series are independently dropped or signals of corresponding wavelength added, and the signals of the two series are then combined.
According to a first aspect, the present invention regards a device for adding and dropping optical signals into and from an optical transmission path, comprising input means for at least two optical signals with distinct wavelengths, characterized by the fact of comprising:
spectral selective means for sending at least a first of said signals to a first optical path and for sending into a second optical path at least a second of said signals;
at least a wavelength selective switching means, along at least one of said first or second optical paths, for adding and dropping at least one optical signal of predetermined wavelength, selected from the wavelengths of said optical signals.
In one version of this invention, said spectral selective means includes an optical circulator and a selective reflection circuit for each of said optical paths. Said selective reflection circuit may include at least one Bragg grating filter.
The wavelengths of said first and second optical signals may be separated by a value lower than the spectral resolution of said selective switching means.
According to another aspect, the present invention concerns a multichannel optical telecommunication system for the transmission of optical signals, comprising:
a fiber-optic line;
means for inputting to said line optical signals with distinct wavelengths separated by a quantity greater than or equal to a minimum predetermined value;
an adding/dropping unit comprising a selective switching means of predetermined spectral resolution, associated with a respective optical path in said unit, and spectral selection means associated with said optical path, characterized by the fact that said minimum value is lower than said predetermined spectral resolution.
According to a third aspect, this invention concerns a method for adding/dropping optical signals, comprising the steps of:
inputting two optical signals of contiguous wavelengths to two separate optical paths;
dropping at least one of said two signals from said optical path and/or adding at least one optical signal with the corresponding wavelength to said optical path.